1. Field of the Invention
The present invention relates to a data printing apparatus and, more particularly, to a data printing apparatus for causing a light-emitting element to emit light in synchronism with traveling of a film to print data such as a date on a traveling film surface.
2. Description of the Related Art
In a conventional camera, as an apparatus for causing a light-emitting element to emit light in synchronism with traveling of a film to print data such as a date on a traveling film surface, apparatuses described in, e.g., Published Examined Japanese Patent Application No. 57-55131 and Published Unexamined Japanese Patent Application No. 60-166938 are known.
The apparatus described in Published Examined Japanese Patent Application No. 57-55131 is provided with a 1-digit light-emitting LED for printing data. When a film is wound up, the LED emits light in correspondence with each digit at proper timings in the order of "month, day, hour, minute, and second", thereby printing data. This apparatus comprises a photocoupler for detecting the rotational amount of a rotational shaft of a sprocket meshing with perforations of a film. Thus, when the film travels by a predetermined length, a light emission signal is output. For this reason, the interval between adjacent characters can be prevented from varying.
In the apparatus described in Published Unexamined Japanese Patent Application No. 60-166938, seven LEDs are juxtaposed in an array in a direction perpendicular to the traveling direction of a film, and these LEDs are turned on five times during film traveling so as to form a character pattern consisting of 5.times.7 dots, thereby printing data. In this apparatus, whether or not the film moving speed is stabilized is judged based on the value of a current flowing through a film feed motor, and an ON/OFF interval is determined on the basis of the stabilized film moving speed. For these reasons, a character having a constant dot interval can be printed.
When the apparatus described in published Examined Japanese Patent Application No. 57-55131 is applied to a camera with a self development function, a sheet film used in the camera with the self development function has no perforations. Therefore, in this case, as described above, a mechanism for detecting the rotational amount of a rotational shaft of a sprocket meshing with the perforations cannot be used.
On the other hand, in a camera using a normal film with perforations, in order to feed a film by one frame, control for, when eight perforations are detected, stopping a film feed motor is normally performed. However, when the above-mentioned printing apparatus is further applied to this camera, an apparatus for detecting the traveling length of a film with high resolution is required in addition to the perforation mechanism. A camera having the above-mentioned printing apparatus must simultaneously and precisely perform feedback control for feeding a film by one frame, and feedback control for performing a data printing operation. For this reason, the arrangement of a control circuit is complicated.
In the apparatus described in Published Unexamined Japanese Patent Application No. 60-166938, the moving speed of a film is detected based on the value of a current flowing through the motor. As a practical problem, the relationship between the rotational speed of the motor and the motor current value varies depending on individual motors. For this reason, it is difficult to detect the film moving speed from the motor current value. If this apparatus is used, the size and interval of characters to be printed on a film vary in individual cameras.